Hafler 9130 feedback loop MOD question - diyAudio
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Old 4th June 2008, 08:02 PM   #1
reiver is offline reiver  United States
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Default Hafler 9130 feedback loop MOD question

I have a Hafler 9130 I'm modding with new PP film caps, electrolytics, bypass caps etc.

The feedback loop is very simple compared to other Hafler amps. It uses just a 3pf cap and a 100mf/16v non-polar cap to ground. Other Haflers use a 470mf np cap here, while a popular mod is to use a 1000mf np cap in this spot.

My question is: 100mf seems awfully small. Would using a 470 or 1000mf cap here on this feedback loop cause any issues? Or would it simply open up the low end response?

See the attached image............

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Old 4th June 2008, 08:45 PM   #2
Zen Mod is offline Zen Mod  Serbia
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1/(2xPi x 100^-6F x1100E) seems already pretty low ;

you'll not gain much , going to more than 220uF ;
place nice film cap (1uF) across ....... and enjoy
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Old 5th June 2008, 01:12 AM   #3
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it's the DC blocker for the feedback loop. it also sets the LF -3db point of the amp's frequency response, usually at about 5hz. it protects woofers from LF excursions caused by things like warped vinyl records (those big black discs that my kids call "huge CD's"). as long as you aren't using media that can cause such LF signals, it's ok to put a 1000uf in there, but be aware that you'll change the amp's -3db point from 5hz to 0.5 hz, really not neccesary. the other cap (the 3pf) sets the amp's HF -3db point, and usually is used if a design has a tendency towards oscillation. leave this cap as-is.
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Old 5th June 2008, 02:34 AM   #4
reiver is offline reiver  United States
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Default re:

Thanks.........good info.

I agree, lowering it from 5hz to .5 isn't worth the soldering time. I noticed that my 9130 was bass-shy compared to my XL280 and was focusing on that cap as the culprit.

I suspect it's the compact 6800mf power supply caps and smaller transformer in the main supply that are more the real cause.

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